1. Field of the Invention
The present invention relates to a process for the preparation of very pure, aqueous solutions of free hydroxylamine.
2. Description of the Related Art
Very pure, concentrated, aqueous hydroxylamine solutions are used, inter alia, in the electronics industry, for example in combination with other substances for preliminary cleaning of the circuit boards. For use in the electronics industry, concentrations of the impurities, in particular metal ions, well below 1 ppm, ie. electronic grade products, are usually required. However, the aqueous hydroxylamine solutions commercially available at present contain impurities in the ppm range from the preparation, for example sodium sulfate or other metal compounds.
Hydroxylamine is produced on a large industrial scale as hydroxylammonium salt, usually as hydroxylammonium sulfate. Frequently, however, it is necessary to use a highly concentrated salt-free aqueous solution of free hydroxylamine. In order to avoid the abovementioned problems and in particular the instability of the hydroxylamine, those skilled in the art have avoided the use of traditional methods of large-scale chemistry for concentrating distillable substances, for example distillation, in the recovery of salt-free hydroxylamine solutions. The distillation of hydroxylamine, even on the laboratory scale, is even said to be a particularly dangerous operation; cf. Roth-Weller: Gefahrliche Chemische Reaktionen, Stoffinformationen Hydroxylamin, page 3, 1984, 2, Ecomed-Verlag. The distillation of hydroxylamine on an industrial scale has therefore also never been considered in technical publications. Instead, special methods have been used, although all of them have serious disadvantages.
Attempts were thus made to isolate free hydroxylamine from aqueous salt solutions with the aid of ion exchangers; cf., for example, U.S. Pst. No. 4,147,623, EP-A-1787, EP-A-237 052 and Z. Anorg. Ch. 288, 28-35 (1956). However, such a process leads only to dilute solutions with low space-time yields. Moreover, hydroxylamine reacts with many ion exchangers or is decomposed by them.
A further method comprises the electrodialysis of an aqueous hydroxylammonium salt solution in electrolysis cells with semipermeable membranes, as described, for example, in DE-A-33 47 259, JP-A-123 771 and JP-A-123 772. However, such a process is technically complicated and expensive and has to date not become established in industry.
DE-A-35 28 463 discloses the preparation of free hydroxylamine from hydroxylammonium sulfate by treatment with calcium oxide, strontium oxide or barium oxide and removal of the insoluble alkaline earth metal sulfates. In this method, the removal of the sulfates obtained in finely divided form presents considerable difficulties. In addition, only dilute solutions are obtained and, when calcium oxide or calcium hydroxide is used, free hydroxylamine still contains undesirably large amounts of ions owing to the relatively good solubility of the calcium sulfate. When strontium compounds and barium compounds are used, the relatively high price and especially the toxicity are moreover disadvantageous with regard to an industrial production process.
DE-A-12 47 282 describes a process in which alcoholic solutions of free hydroxylamine are obtained by reacting hydroxylammonium sulfate with ammonia in alcohol as a solvent and removing the ammonium sulfate. A similar process is described in EP-A-108 294. However, alcoholic solutions are unsuitable and undesirable for a number of applications. For example, particular precautions must be taken during the handling of such solutions, owing to their flammability. Furthermore, the alcohol used must as a rule be recovered by an expensive procedure, since the discharge of relatively large amounts of alcohol into waste water treatment plants or into outfalls is prohibited.
Finally, DE-A-36 01 803 describes a process for obtaining aqueous solutions of free hydroxylamine, in which hydroxylammonium sulfate is reacted with ammonia in lower alcohols, the precipitated ammonium sulfate is separated off, water is added to the alcoholic solution of free hydroxylamine and the alcohol is distilled off from the solution thus obtained. The abovementioned disadvantages of working with alcohol are applicable to this process too. Moreover, owing to the instability of the hydroxylamine in conjunction with the flammability of the alcohols, particular caution is required in the final distillation stage.
Common to all prior art processes is that they are not suitable for being carried out on an industrial scale or give rise to uneconomically high additional safety costs.
For the decomposition of hydroxylamine, a temperature above 65.degree. C. is regarded as critical. In a differential thermal analysis, the onset temperature of a 50% strength by weight aqueous hydroxylamine solution (in a glass crucible) was determined as 70.degree. C. The amount of heat liberated, viz. about 2.2 kJ/g of 50% strength by weight solution, confirms the high thermal potential of the material. Differential thermal analysis is a microthermoanalytical method which is employed for screening to estimate the thermal stability and the thermal potential. The onset temperature is the lowest ambient temperature at which a noticeable exothermic reaction proceeds in the sample at a heating rate of 1 K/min, commencing at 30.degree. C. For safety reasons, the processing temperature should be significantly below the onset temperature.
In the context of the preparation of hydroxylamine nitrate, U.S. Pat. No. 4,956,168 describes the preparation of a slurry of hydroxylamine sulfate in alcohol at a temperature which does not exceed 65.degree. C. This slurry is then treated with ammonia at .ltoreq.65.degree. C. to produce an alcoholic hydroxylamine solution.
U.S. Pat. No. 5,472,679 describes a process for preparing an alcohol-free, aqueous hydroxylamine solution by reacting a hydroxylamine sulfate solution with a suitable base at up to about 60.degree. C. The mixture obtained is then subjected to distillation under reduced pressure at below 65.degree. C. This gives a solid residue (the salt formed in the liberation of the hydroxylamine) and as distillate an aqueous hydroxylamine solution containing 16-23% by weight of hydroxylamine. This process has the disadvantage that it requires working under reduced pressure and the temperature has to be controlled carefully.
In addition, the process requires working with solids. In a continuous process, the solid would accordingly have to be removed continuously. This can present great problems in terms of process technology if the solid is one which tends to cake, eg. in the case of Na.sub.2 SO.sub.4 xH.sub.2 O.
Furthermore, the "distillation" proceeds to dryness, more correctly described as evaporation, such that the low-boiling water evaporates first. The high-boiling hydroxylamine accumulates. It is known that the decomposition tendency of hydroxylamine increases with the concentration of hydroxylamine, and together with it the losses of hydroxylamine during the process. There is an increasing risk that, because of the high concentration of hydroxylamine, explosive decomposition will occur. It is known that pure hydroxylamine or hydroxylamine&gt;70% by weight decomposes explosively. Thus, appropriate safety requirements must be fulfilled for the process mentioned.
Finally, the remaining solid still contains residues of hydroxylamine (hydroxylamine adsorbed on the surface, hydroxylamine in interstitial spaces in the solid). The solid therefore has to be decontaminated in a separate disposal process.
DE 1954775.8 describes a process for the preparation of aqueous solutions of free hydroxylamine, the solution obtained by treating a hydroxyl ammonium salt with a base being separated into an aqueous hydroxylamine fraction and a salt fraction by treatment with water or steam at &gt;80.degree. C. Any desired concentration of the aqueous hydroxylamine solution obtained is carried out by distillation, by evaporating off water in a column. In addition to the hydroxylamine, the sparingly volatile impurities may also accumulate in the bottom. This problem which is a general one in the case of bottom products is solved in industry, for example, by a further distillation. In the case of hydroxylamine, however, this is problematic since concentration of hydroxylamine to over 50% by weight is unavoidable in a further distillation of the, for example, 50% strength solution. However, the tendency of the hydroxylamine to undergo decomposition also greatly increases. The distillation must therefore be carried out at low temperatures and pressures at a corresponding cost and with a corresponding time requirement, and can usually also be effected only on a small scale. Accordingly, salt-free, aqueous hydroxylamine solutions of electronic grade purity are complicated to prepare and therefore relatively expensive and, for economic reasons, their use is restricted to a few areas.
It is an object of the present invention to provide a simple process for the preparation of very pure hydroxylamine containing &lt;1 ppm of metal ions.